How do I use an already running Emacs from another window?

emacsclient, which comes with Emacs, is for editing a file using an already running Emacs rather than starting up a new Emacs. It does this by sending a request to the already running Emacs, which must be expecting the request.

• Setup:

  Emacs must have executed the server-start function for ‘emacsclient’ to work. This can be done either by a command line option:

  emacs -f server-start

  or by invoking server-start from init file (see How do I set up an init file properly?):

  (if (some conditions are met) (server-start))

  To get your news reader, mail reader, etc., to invoke ‘emacsclient’, try setting the environment variable EDITOR (or sometimes VISUAL) to the value ‘emacsclient’. You may have to specify the full pathname of the ‘emacsclient’ program instead. Examples:

  # csh commands:
  setenv EDITOR emacsclient
  
  # using full pathname
  setenv EDITOR /usr/local/emacs/etc/emacsclient
  
  # sh command:
  EDITOR=emacsclient ; export EDITOR

• Normal use:

  When ‘emacsclient’ is run, it connects to the socket and passes its command line options to Emacs, which at the next opportunity will visit the files specified. (Line numbers can be specified just like with Emacs.) The user will have to switch to the Emacs window by hand. When the user is done editing a file, the user can type C-x # (or M-x server-edit) to indicate this. If there is another buffer requested by emacsclient, Emacs will switch to it; otherwise emacsclient will exit, signaling the calling program to continue.

For more information, See Emacs Server in The GNU Emacs Manual.